18,862 research outputs found
Accretion Disk Temperatures of QSOs: Constraints from the Emission Lines
QSO emission-line spectra are compared to predictions based on theoretical
ionizing continua of accretion disks. Observed line intensities do not show the
expected trend of higher ionization with higher accretion disk temperature as
derived from the black hole mass and accretion rate. This suggests that, at
least for accretion rates close to the Eddington limit, the inner disk does not
reach temperatures as high as expected from standard disk theory. Modified
radial temperature profiles, taking account of winds or advection in the inner
disk, achieve better agreement with observation. This conclusion agrees with an
earlier study of QSO continuum colors as a function of disk temperature. The
emission lines of radio-detected and radio-undetected sources show different
trends as a function of disk temperature.Comment: 9 pages, 8 figures, submitted to Ap
Speech Communication
Contains reports on two research projects.United States Air Force, Electronic Systems Division (Contract AF19(628)-3325)National Science Foundation (Grant G-16526)National Institutes of Health (Grant MH-04737-03 and Grant NB-04332-01)National Aeronautics and Space Administration (Grant NsG-496
Time scale of forerunners in quantum tunneling
The forerunners preceding the main tunneling signal of the wave created by a
source with a sharp onset or by a quantum shutter, have been generally
associated with over-the-barrier (non-tunneling) components. We demonstrate
that, while this association is true for distances which are larger than the
penetration lenght, for smaller distances the forerunner is dominated by
under-the-barrier components. We find that its characteristic arrival time is
inversely proportional to the difference between the barrier energy and the
incidence energy, a tunneling time scale different from both the phase time and
the B\"uttiker-Landauer (BL) time.Comment: Revtex4, 14 eps figure
Spitzer/MIPS Limits on Asteroidal Dust in the Pulsar Planetary System PSR B1257+1
With the MIPS camera on Spitzer, we have searched for far-infrared emission
from dust in the planetary system orbiting pulsar PSR 1257+12. With accuracies
of 0.05 mJy at 24 um and 1.5 mJy at 70 um, photometric measurements find no
evidence for emission at these wavelengths. These observations place new upper
limits on the luminosity of dust with temperatures between 20 and 1000 K. They
are particularly sensitive to dust temperatures of 100-200 K, for which they
limit the dust luminosity to below of the pulsar's spin-down
luminosity, three orders of magnitude better than previous limits. Despite
these improved constraints on dust emission, an asteroid belt similar to the
Solar System's cannot be ruled out
Sensitivity of surface fluxes in the ECMWF land surface model to the remotely sensed leaf area index and root distribution: Evaluation with tower flux data
The surface-atmosphere turbulent exchanges couple the water, energy and carbon budgets in the Earth system. The biosphere plays an important role in the evaporation process, and vegetation related parameters such as the leaf area index (LAI), vertical root distribution and stomatal resistance are poorly constrained due to sparse observations at the spatio-temporal scales at which land surface models (LSMs) operate. In this study, we use the Carbon Hydrology Tiled European Center for Medium-Range Weather Forecasts (ECMWF) Scheme for Surface Exchanges over Land (CHTESSEL) model and investigate the sensitivity of the simulated turbulent fluxes to these vegetation related parameters. Observed data from 17 FLUXNET towers were used to force and evaluate model simulations with different vegetation parameter configurations. The replacement of the current LAI climatology used by CHTESSEL, by a new high-resolution climatology, representative of the station’s location, has a small impact on the simulated fluxes. Instead, a revision of the root profile considering a uniform root distribution reduces the underestimation of evaporation during water stress conditions. Despite the limitations of using only one model and a limited number of stations, our results highlight the relevance of root distribution in controlling soil moisture stress, which is likely to be applicable to other LSMs
Sources of quantum waves
Due to the space and time dependence of the wave function in the time
dependent Schroedinger equation, different boundary conditions are possible.
The equation is usually solved as an ``initial value problem'', by fixing the
value of the wave function in all space at a given instant. We compare this
standard approach to "source boundary conditions'' that fix the wave at all
times in a given region, in particular at a point in one dimension. In contrast
to the well-known physical interpretation of the initial-value-problem
approach, the interpretation of the source approach has remained unclear, since
it introduces negative energy components, even for ``free motion'', and a
time-dependent norm. This work provides physical meaning to the source method
by finding the link with equivalent initial value problems.Comment: 12 pages, 7 inlined figures; typos correcte
Synchrotron flaring behaviour of CygnusX-3 during the February-March 1994 and September 2001 outbursts
Aims: In this paper we study whether the shock-in-jet model, widely used to
explain the outbursting behaviour of quasars, can be used to explain the radio
flaring behaviour of the microquasar Cygnus X-3.
Method: We have used a method developed to model the synchrotron outbursts of
quasar jets, which decomposes multifrequency lightcurves into a series of
outbursts. The method is based on the Marscher & Gear (1985) shock model, but
we have implemented the modifications to the model suggested by Bjornsson &
Aslaksen (2000), which make the flux density increase in the initial phase less
abrupt. We study the average outburst evolution as well as specific
characteristics of individual outbursts and physical jet properties of Cyg X-3.
Results: We find that the lightcurves of the February-March 1994 and
September 2001 outbursts can be described with the modified shock model. The
average evolution shows that instead of the expected synchrotron plateau, the
flux density is still increasing during the synchrotron stage. We also find
that high frequency peaking outbursts are shorter in duration than the ones
peaking at lower frequencies. Finally, we show that the method can be used,
complementary to radio interferometric jet imaging, for deriving the physical
parameters such as the magnetic field strength and the energy density of
relativistic electrons in the jet of Cyg X-3.Comment: 8 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Role of Interfaces in the Proximity Effect in Anisotropic Superconductors
We report measurements of the critical temperature of YBCO-Co doped YBCO
Superconductor-Normal bilayer films. Depending on the morphology of the S-N
interface, the coupling between S and N layers can be turned on to depress the
critical temperature of S by tens of degrees, or turned down so the layers
appear almost totally decoupled. This novel effect can be explained by the
mechanism of quasiparticle transmission into an anisotropic superconductor.Comment: 13 pages, 3 figure
The sound of violets: the ethnographic potency of poetry?
This paper takes the form of a dialogue between the two authors, and is in two halves, the first half discursive and propositional, and the second half exemplifying the rhetorical, epistemological and metaphysical affordances of poetry in critically scrutinising the rhetoric, epistemology and metaphysics of educational management discourse.
Phipps and Saunders explore, through ideas and poems, how poetry can interrupt and/or illuminate dominant values in education and in educational research methods, such as:
âą alternatives to the military metaphors â targets, strategies and the like â that dominate the soundscape of education;
âą the kinds and qualities of the cognitive and feeling spaces that might be opened up by the shifting of methodological boundaries;
âą the considerable work done in ethnography on the use of the poetic: anthropologists have long used poetry as a medium for expressing their sense of empathic connection to their field and their subjects, particularly in considering the creativity and meaning-making that characterise all human societies in different ways;
âą the particular rhetorical affordances of poetry, as a discipline, as a practice, as an art, as patterned breath; its capacity to shift phonemic, and therewith methodological, authority; its offering of redress to linear and reductive attempts at scripting social life, as always already given and without alternative
The Structure of p-Aminobenzoic Acid in Water: Studies Combining UV-Vis, NEXAFS and RIXS Spectroscopies
NEXAFS-RIXS and home laboratory-based UV-Vis absorption spectroscopy are combined to examine the speciation and electronic structure of para-aminobenzoic acid (PABA) in aqueous solution as a function of pH. DFT and TD-DFT electronic structure calculations reproduce the experimental trends and provide a correlation between the experimental HOMOâLUMO gap as well as the electronic transitions between molecular orbitals in the non-ionic, anionic and cationic forms of PABA
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